Remove dead code from the powerpc uma_small_alloc().

32-bit Book-E doesn't set UMA_MD_SMALL_ALLOC, and 32-bit OEA platforms
have a 32-bit vm_paddr_t. Moreover, this code was wrong in that it
leaked the page if the check failed.

Reviewed by: jhibbits
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23991